Single crystal silicon, which is the starting material in most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon, and then a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by, for example, decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter is typically reduced gradually to form a tail end in the form of an end-cone. The end-cone usually is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
Solar cells may be fabricated from monocrystalline silicon substrates produced by the Czochralski method. Czochralski grown monocrystalline silicon substrates may be grown by either standard, i.e., batch, or continuous. In order to achieve acceptable resistivity for solar cell applications, the growing crystal is doped primarily with boron. It is the industry standard for diffused junction screen-printed solar cells to use boron-doped silicon wafers.
The use of boron-doped silicon wafers is not without problems. For example, it is known that oxygen impurity, commonly caused by the crucible, of a Czochralski grown crystal can interact with boron dopants forming complexes in the material. These oxygen complexes are activated when the substrate or finished solar cell is exposed to light, which degrades its minority carrier lifetime, and hence the efficiency of the completed solar cell. This phenomenon is called light induced degradation (LID), and is a major loss mechanism for solar cells fabricated on boron-doped monocrystalline silicon wafers.
In order to minimize the effect of LID, manufacturers target a slightly higher resistivity than optimal to reduce the amount of boron dopant atoms in each wafer. Therefore, there is a trade-off between LID and optimal base resistivity. Consequently, the maximum efficiency of solar cell cannot be realized.